By use of a through-flow freeze for this purpose, an ice layer is formed on the inner wall of the freezing cylinder of the through-flow freezer, which has to be scraped off by means of the conveyor screw that is rotating in the freezing cylinder and at the same time creates a positive conveyance of the ice cream mass through the freezing cylinder. The result of the use of such a known conveyor screw freezer for the further cooling down is, however, not positive as, in order to carry out the scraping off and the conveyance of the ice cream mass, it is necessary to add such a quantity of power that the freezer will become inefficient due to the added scraping, kneading and pumping power, which results in a generation of heat in the ice cream mass. This generation of heat is counteracted by using further low temperatures for freezing ice on the inner wall of the cylinder with the result that the formation of the ice layer is increased so that again more power is required in order to perform the scraping function.
Through-flow freezers, which work in this way, are known from the literature, see DE-A-39 18 268 and WO 97/26800, but are rarely used in practice as far as ordinary ice cream is concerned as it has been found that the process involves quite essential problems. In a principal level, the process type is most attractive as it ideally would make it possible to form and pack ice bodies directly for the final storage without the conventional use of a temporary and expensive freezing system between the packing station and the final storage. Moreover, an intensive cooling of the mass will enable an improved product quality, in particular when producing larger block products.
The starting point of the invention is a conventional through-flow freezer having a driven, scraping conveyor screw dimensioned for further conveyance of the flow from the preceding conventional through-flow freezer which cools the mass down to approximately −5° C. A standard mix of ice cream with a so-called overrun (degree of swelling) of 100% was used, i.e. the volume of the air added the liquid mix in percentage of the volume of the liquid mix, and in the subsequent through-flow freezer, which should cool the ice cream from the first through-flow freezer further down to approx. −15° C., an evaporation temperature of approximately −40° C. was used.
The higher viscosity of the ice cream in the subsequent through-flow freezer due to the lower temperature caused a number of problems. The mechanical power, which must be used in order to drive the conveyor screw around, becomes substantially higher than for the preceding through-flow freezer where the temperature of the ice cream mass is higher and the motor driving the conveyor screw therefore has to yield a larger torque. The mechanical power is deposed in the ice cream mass which therefore must be cooled further and cause a higher power consumption for cooling, as well as a limitation of the capacity of the subsequent through-flow freezer. In addition, the inlet pressure of the subsequent through-flow freezer must be high, in the order of 12-14 bar, in order for the ice cream mass to be fed to it, which poses heavy demands on pipes, gaskets etc. strains the preceding through-flow freezer, which must deliver the high pressure as well as reduces the quality of the product as the high pressure causes a lowering of the melting point of the ice cream mass which promotes the development of ice crystals in the ice cream mass and reduces the overrun of the ice cream. Further, experiments showed that the discharge quantity per time unit for the subsequent through-flow freezer displayed fluctuations, which is inexpedient for the further manufacturing process for the ice cream mass, and that the degree of swelling decreased form inlet to outlet during creation of larger air pockets in the ice cream mass.
It is therefore an object of the present invention to provide a conveyor screw and a conveyor with such a conveyor screw, which can convey high viscose masses such as ice cream mass by use of a lower conveyance power.
It is a further object of the invention to provide such a conveyor screw where the inlet suction at the inlet end is increased so that the in-feed pressure may be reduced.
Furthermore, it is an object of the present invention to provide an apparatus for making ice cream comprising a through-flow freezer with a conveyor screw which can convey high viscose ice cream mass by use of a reduced conveyance power compared to known apparatuses with conveyor screws.
It is yet another object of the invention to provide such an apparatus where the inlet suction at the inlet end of the conveyor screw is increased so that the feed-in pressure may be reduced compared to known apparatuses.
In general, it is an object of the invention to improve the efficiency of such a conveyor screw and such an apparatus and facilitate or eliminate one or more of the disadvantages described above.
By the invention a conveyor screw having a plurality of screw flights is provided where at least two screw flights extend from an inlet end part of the conveyor screw, wherein the outer edges of the two screw flights extend in different radial distance from the longitudinal axis. Thereby, the radially shorter screw flight assists with the conveyance of a part of the mass without affecting the whole mass and especially without affecting the ice layer, which by use of the conveyor screw in a through-flow freezer is created on the inner wall of the freezing cylinder. It has shown that by conveyance of a high viscose mass, a lower conveyance power is used by this type of conveyor screw than by any other known type. The effect is especially shown by use in a through-flow freezer which further cools down an already frozen ice cream mass, but is also shown by conveyance of other high viscose masses where the resistance against conveyance of the mass is highest at the cylinder wall where the mass attaches. The inlet pressure by the through-flow freezer, which was used for the experiment, could be reduced to 4-6 bar compared with 12-14 bar earlier.
Conveyor screws with several screw flights of which some have a shorter radial extent are known from extruder machines as described in U.S. Pat. No. 3,701,512, GB 2 262 905, U.S. Pat. Nos. 6,132,075 and 5,141,326 where these screw flights are arranged in a compressing or outlet zone with quite another effect than the one achieved with the present invention.
Fulfilment of the other objects of the invention and other advantages thereby will appear from the following description.